In fabricating integrated circuits multiple patterns are commonly transferred each from a mask or reticle into a resist layer formed on a semiconductor wafer by means of an exposure in an exposure tool. Each of the exposures is usually followed by steps of processing the layer, which may relate to developing the resist, transferring the pattern formed in the resist further into an underlying layer, removing the resist, etc., such that one of a number of levels of the integrated circuit is formed.
With continuously increasing structure densities within patterns the resolution capability of exposure tools has been reached, which happens when structure widths become comparable to the wavelength of a corresponding exposure light beam. Accordingly, resolution enhancement techniques have been developed to push the critical dimensions of structures to even smaller dimensions. These techniques relate, for example, to the use of half-tone or alternating phase shift masks in an exposure, to an application optical proximity correction, when the mask layout is created, etc.
Typical patterns to be transferred from a mask to a semiconductor wafer having small and thus critical structure widths are contact hole patterns. These are used, e.g., to connect different metal layers of the integrated circuit. In the case of dynamic random access memories (DRAMs), contact hole patterns are provided with a highly regular or periodic arrangement of square-like structures, where each hole pattern has a particularly critical width with respect to the resolution limit of a respective exposure tool.
In comparison with lines and spaces patterns, contact hole patterns are less suited for the alternating phase shift mask technique. Consequently, the process window defined by allowed ranges of exposure tool parameters, such as dose and focus (which yield a sufficient dimensional stability or accuracy of the transferred pattern with respect to the original formed on the mask), is considerably reduced. Nevertheless, the same arguments hold true for any other pattern that has structure widths near the practical resolution limit. Further important examples are patterns comprising, e.g., both isolated and periodically arranged structures.
There have also been extensive efforts to improve the exposure tools used to perform an exposure. These efforts had been concentrated on increasing the depth of focus (i.e., the range of focus settings which yield a structure width) which is in sufficient proximity to a specified reference width. One known technique is to vary the focus throughout an exposure or, alternatively, to apply multiple exposures with respect to one pattern transferal from a mask onto a wafer with each exposure representing a different value of the focus set for this exposure. The latter method of varying the z-axis of the wafer stage in an exposure tool to discrete focal planes is called focus latitude enhancement exposure (FLEX). The FLEX method is disclosed in Fukuda et al., “Improvement of defocus tolerance in a half-micron optical lithography by the focus latitude enhancement exposure method: simulation and experiment”, J. VAC. SCI. TECHNOL. B. 7 (4) July/August 1989, p. 667-674.
The fundamental idea of this method is to overlay the aerial images due to a number of exposures, each of the exposures being applied with a different value for the focus offset of the wafer plane with respect to a focal plane. This overlay is performed in the same resist layer on a wafer, i.e., within the same exposure process comprising a number of exposure steps. The range of focus settings is increased, which yields a specified range of structure widths for the critical structures of the pattern, i.e., the depth of focus (DOF) is improved. Generally, this improvement arrives at the cost of reductions in image contrast, among others. However, the improvement of an increased process window more than completely outweighs that reduction in contrast.
A still further refinement of the method is achieved by applying a tilt in y- or x-direction of the wafer stage during exposure. The wafer is then scanned with an exposure slit while changing the local focus due to a z-movement of the stage. As a result, a through-the-focus exposure during just one exposure or scanning step becomes feasible.
However, there are some disadvantages of both types of multiple focus exposures: In the case of applying an x- or y-tilt to the wafer stage, the dynamic leveling range of the exposure tool wafer stage is limited. Further, a trade-off exists between the focus offset control and the illumination uniformity across the slit, which scans the tilted wafer surface.
The multiple exposure technique according to FLEX suffers from throughput impacts, since the scanning slit needs some time when repeatedly crossing the exposure field and wafer surface. Further, discrete focus adjustments have to be performed, where with each adjustment an additional autofocus error is introducing amounting to +/−25 nm.
In order to circumvent the problems of mechanical focus adjustments of the wafer stage, another approach is proposed which mimics a focus adjustment by means of varying the wavelength of the exposure tool light source which is used to illuminate the mask or reticle for transferring the pattern onto the wafer. The method is also called “resolution enhancement by laser-spectrum adjusted exposure” (RELAX), and is disclosed, e.g., in Lalovic, I. et al., “Depth of focus enhancement by wavelength modulation: can we RELAX and improve focus latitude?”, (Proceedings of 40th Interface Symposium Conference 2003, San Diego, Sep. 21-23, 2003), and also in U.S. Patent Application Publication No. US 2002/0048288. In the RELAX method, a linear relationship exists between defocus (focus offset from ideal or best focus) and wavelength such that a modulation of a component of the output spectrum of a laser light source in an exposure tool corresponds to a change in a focus profile. This can be seen from the following relation:
      DOF    =                  k        2            *              λ                  2          *                      (                          1              -                                                1                  -                                      NA                    2                                                                        )                                ,where the paraxial approximation has been removed. In this equation, DOF refers to the depth of focus, λ to the wavelength of the exposure light, NA is the numerical aperture of the lens system, and k2 is determined to +/−0.5.
As a consequence, it is possible to achieve one multifocus exposure by means of, e.g., a dual wavelength beam. For example, an ArF-laser, emitting light at 193 nm, can be tuned to effectively mimic a double exposure at two different focus values, wherein the intensity spectrum comprises essentially two intensity peaks, dislocated from each other by, e.g., 0.5 pm-4.0 pm in wavelength.
However, the dual wavelength approach also suffers from disadvantages. In particular, there is a variation of chromatic magnification across an image field, which might lead to a local image displacement at the borders of the image field. This variation is due to the optical dispersion behavior of the lens system implemented in the exposure tools as well as chromatic aberration of the lenses (higher order aberration terms).
One solution is to compensate chromatic aberration by employing two different materials for the lenses, one having positive and the other negative dispersion function (dn/dλ). It has been shown that, for lenses in 248 nm exposure tools, this combination of materials is quite effective in reducing chromatic aberration.